1. Field of the Invention
The present invention relates to a variable optical wavelength filter for selecting the wavelength of an optical output signal from the wavelength of an optical input signal and outputting the optical output signal.
2. Description of Related Art
Conventionally, dielectric multilayer filters are used in variable optical wavelength filters in the field of optical interconnections. The wavelength of the optical input signal input to this dielectric multilayer filter may vary because of temperature variations in the oscillator generating this optical input signal. It is normally necessary to substantially maximize the transmittance of the dielectric multilayer filter with respect to the optical output signal corresponding to this wavelength variation. For this reason, before now, the filter was controlled and caused to follow this wavelength variation, so that the dielectric multilayer filter comes to a position at which the maximum transmittance is substantially provided, with the axis of rotation being the center line of the dielectric multilayer filter, orthogonal to the crossover line of the incident plane at which the optical input signal is incident to the dielectric multilayer filter and the dielectric multilayer filter.
However, there are two problems with a variable optical wavelength filter having a conventional constitution. The first problem is that it becomes impossible to control the rotation of the dielectric multilayer filter in the event of a momentary interruption of the optical input signal (caused by a lightning strike, for example). The second problem is that it becomes impossible to control the rotation of the dielectric multilayer filter when the frequency of wavelength variation of the optical input signal discussed above becomes a specific frequency (detection frequency). The desired optical output signal cannot be branched or outputted from the dielectric multiplayer filter when it becomes impossible to control the rotation of the dielectric multilayer filter.
In order to resolve the first problem, it is desirable to develop a variable optical wavelength filter that can retain the position of the optical wavelength filter even in the event of a momentary interruption of the optical input signal.
In order to resolve the second problem, it is desirable to develop a variable optical wavelength filter with which the movement of the optical wavelength filter can be controlled regardless of a variations in intensity of the optical input signal.
In order to resolve the first problem, the variable optical wavelength filter according to a first aspect of the present invention is provided an optical wavelength filtering portion and optical wavelength filter control system, for a constitution for selecting the wavelength of the optical output signal from the wavelengths of the optical input signal and outputting or branching this optical output signal.
This optical wavelength filtering portion comprises an optical wavelength filter for selecting the specific wavelength for providing the maximum transmission peak to the optical output signal from the wavelengths of the optical input signal.
This optical wavelength filter control system outputs to the optical wavelength filtering portion a control signal comprising: a direct current movement signal for moving the wavelength filter to the position for selecting the specific wavelength of the optical output signal according to the wavelength variation of the optical input signal; and an alternating current detection signal for providing the amplitude frequency to the optical output signal by causing the optical wavelength filter to vibrate slightly at a fixed detection frequency.
Furthermore, this optical wavelength filter control system comprises an optical wavelength filter control portion including a direct current component forming portion for sustaining the output of the abovementioned direct current movement signal even in the event of a momentary loss of the optical input signal, and determining the size of the direct current movement signal based on the amplitude of the optical output signal; and an alternating current detection signal generator for outputting the abovementioned alternating current detection signal of the detection frequency.
With the abovementioned variable optical wavelength filter according to the present invention, this optical wavelength filter control portion can output the direct current movement signal for filtering the optical output signal to the optical wavelength filtering portion even in the event of a momentary loss of the optical input signal. Consequently, this optical wavelength filter control portion can maintain the position of the abovementioned optical wavelength filter when there is an optical input signal, of course, and in the same way when the optical input signal is lost.
For realizing the optical variable wavelength filter according to a first aspect of the present invention, the optical wavelength filter control system may preferably comprise a beam splitting portion for splitting the optical output signal into an optical monitoring signal and an optical main signal; and a photoelectric converting portion for converting this optical monitoring signal to an electric feedback monitoring signal and outputting this signal to the abovementioned optical wavelength filter control portion.
With the constitution as described above, the electric feedback monitoring signal having amplitude frequency can be output to the optical wavelength filter control portion.
For realizing the variable optical wavelength filter according to a first aspect of the present invention, the optical wavelength filter control system preferably comprises first and second beam splitting portions, first and second photoelectric converting portions, and a divider or dividing portion, in order to resolve the second problem discussed above.
The first beam splitting portion splits the optical input signal into a first optical monitoring signal and an optical input signal. Also, the second beam splitting portion splits the optical output signal into a second optical monitoring signal and an optical main signal. These first and second photoelectric converting portions convert the first and second optical monitoring signals to first and second electric monitoring signals respectively. Also, this divider divides the second electric monitoring signal by the first electric monitoring signal and outputs the result as the electric feedback monitoring signal to the abovementioned optical wavelength filter control portion.
With the constitution as discussed above, the amplitude of the optical output signal becomes the product of the amplitude of the optical input signal and the transmittance of the abovementioned wavelength filter which vibrates slightly at the detection frequency, in the case where the optical input signal is a single wavelength beam. The components of these optical input and output signals are equivalent respectively to the components of the first and second electric monitoring signals. Consequently, the amplitude of the second electric monitoring signal becomes the product of the amplitude of the first electric monitoring signal and the transmittance of the wavelength filter which vibrates slightly at the detection frequency. Accordingly, by dividing the second electric monitoring signal by the first electric monitoring signal, the divider or dividing portion discussed above can output to the abovementioned optical wavelength filter control portion an electric feedback monitoring signal that is a signal with the component of the first electric monitoring signal removed from the second electric monitoring signal. As discussed above, the component of the first electric monitoring signal is equivalent to the component of the optical input signal. As a result, the component of the optical input signal is not included in the electric feedback monitoring signal output to the optical wavelength filter control portion. Accordingly, even if the intensity of the optical input signal varies, the electric feedback monitoring signal does not include the component resulting from the variations in intensity of the optical input signal. The abovementioned optical wavelength filter control portion extracts the alternating current component from this electric feedback monitoring signal and controls the optical position of the wavelength filter on the basis of that alternating current component. The alternating current component does not include the variable component resulting from the variations in intensity of the optical input signal. Accordingly, this optical wavelength filter control portion can control the optical wavelength filter regardless of the variations in intensity of the optical input signal because of being able to output to the optical wavelength filtering portion a control signal for providing the maximum transmission peak to the optical output signal.
For realizing the variable optical wavelength filter according to a first aspect of the present invention, the direct current component forming portion preferably comprises a lock-in amplifier, an amplitude amplifier, and an integrator (an inverting amplifier, for example).
This lock-in amplifier extracts the amplitude frequency component from the abovementioned electric feedback monitoring signal and smoothes and converts this amplitude frequency component to a first direct current signal. Next, this amplitude amplifier amplifies the amplitude of this first direct current signal and converts this signal to a second direct current signal. Next, this integrator integrates this second direct current signal and outputs the result as the direct current movement signal to the optical wavelength filtering portion.
With the constitution as discussed above, this integrator accumulates and sums these second direct current signals. In other words, this integrator functions as a memory holder for the direct current movement signal that is the cumulative sum of these second direct current signals. Accordingly, even if the second direct current signal input to the integrator is momentarily lost due to a momentary loss of the optical input signal, this integrator continues to output the direct current movement signal, for branching or outputting or filtering optical output signal, to the abovementioned optical wavelength filtering portion during that momentary loss. Consequently, the position of the optical wavelength filter can be retained when the optical input signal is present, of course, and in the same way during a momentary loss of the optical input signal.
Explained next is the variable optical wavelength filter according to a second aspect of the present invention for resolving the second problem and having a separate constitution from the variable optical wavelength filter according to the first aspect.
The variable optical wavelength filter according to a second aspect of the present invention is provided an optical wavelength filtering portion and optical wavelength filter control system, for a constitution for selecting the wavelength of the optical output signal from the wavelengths of the optical input signal and outputting or filtering or branching this optical output signal.
This optical wavelength filtering portion comprises an optical wavelength filter for selecting the wavelength of the optical output signal including a specific wavelength.
In the case where the wavelength of the optical output signal matches a predetermined reference wavelength, this optical wavelength filter selects the wavelength for providing the maximum transmission peak to the optical output signal from the wavelengths of the optical input signal as the specific wavelength. In the case where the wavelength of the optical output signal differs from the reference wavelength, this optical wavelength filter selects the wavelength for providing a transmittance slightly less than the maximum transmission peak, and corresponding to this difference, from the wavelengths of the optical input signal to the optical output signal as the specific wavelength.
Furthermore, the abovementioned optical wavelength filter control system outputs to the optical wavelength filtering portion a control signal comprising a direct current movement signal for moving the optical wavelength filter to the position for selecting the specific wavelength corresponding to the wavelength variations of the optical input signal, and an alternating current detection signal for providing the amplitude frequency to the optical output signal by causing slight vibration of the optical wavelength filter at a fixed detection frequency.
This optical wavelength filter control system comprises a wavelength filter control portion, first and second beam splitting portions, first and second photoelectric converting portions, and a dividing portion or divider.
This optical wavelength filter control portion includes a direct current component forming portion for determining the size of the direct current movement signal based on the amplitude of the optical output signal, and an alternating current detection signal generator for outputting the alternating current detection signal of the detection frequency. The first beam splitting portion splits the optical input signal into a first optical monitoring signal and an optical input signal. The second beam splitting portion splits the optical output signal into a second optical monitoring signal and an optical main signal. The first and second photoelectric converting portions convert the first and second optical monitoring signals to first and second electric monitoring signals respectively. The divider divides the second electric monitoring signal by the first electric monitoring signal and outputs the result as the electric feedback monitoring signal to the abovementioned optical wavelength filter control portion.
With the constitution as discussed above, the amplitude of the optical output signal becomes the product of the amplitude of the optical input signal and the transmittance of the abovementioned optical wavelength filter which vibrates slightly at the detection frequency, in the case where the optical input signal is a single wavelength beam. The components of these optical input and output signals are equivalent respectively to the components of the first and second electric monitoring signals. Consequently, the amplitude of the second electric monitoring signal becomes the product of the amplitude of the first electric monitoring signal and the transmittance of the wavelength filter which vibrates slightly at the detection frequency. Accordingly, by dividing the second electric monitoring signal by the first electric monitoring signal, the dividing portion or divider discussed above can output to the abovementioned optical wavelength filter control portion an electric feedback monitoring signal that is a signal with the component of the first electric monitoring signal removed from the second electric monitoring signal. As discussed above, the component of the first electric monitoring signal is equivalent to the component of the optical input signal. As a result, the component of the optical input signal is not included in the electric feedback monitoring signal output to the optical wavelength filter control portion. Accordingly, even if the intensity of the optical input signal varies, the electric feedback monitoring signal does not include the component resulting from the variations in intensity of the optical input signal. The abovementioned optical wavelength filter control portion extracts the alternating current component from this electric feedback monitoring signal and controls the position of the optical wavelength filter on the basis of that alternating current component. The alternating current component does not include the variable component resulting from the variations in intensity of the optical input signal. Accordingly, this optical wavelength filter control portion can control the optical wavelength filter regardless of the variations in intensity of the optical input signal because of being able to output to the optical wavelength filtering portion a control signal for providing the maximum transmission peak to the optical output signal.
Also, for realizing the variable optical wavelength filter according to a second aspect of the present invention, the direct current component forming portion preferably comprises a reference bias component generator, a lock-in amplifier, an amplitude amplifier, and an adder.
This reference bias component generator outputs a reference bias signal for moving the optical wavelength filter to the standard position for selecting the reference wavelength. This lock-in amplifier extracts the amplitude frequency component from the abovementioned electric feedback monitoring signal and smoothes and converts this amplitude frequency component to a first direct current signal. Also, this amplitude amplifier amplifies the amplitude of this first direct current signal and converts this signal to a second direct current signal for moving this optical wavelength filter from the abovementioned standard position to a position for selecting the specific wavelength. Also, this adder adds the second direct current signal and the reference bias signal and outputs the result as the abovementioned direct current movement signal to the optical wavelength filtering portion.
With the constitution as discussed above, the direct current component forming portion can output the direct current movement signal for filtering the optical output signal to the optical wavelength filtering portion.
Also, for realizing the variable optical wavelength filter according to the first or second aspect of the present invention, the optical wavelength filter may preferably be a dielectric multilayer filter.
If a dielectric multilayer filter is used as the optical wavelength filter, the wavelength vs. transmittance curve (hereinafter xe2x80x9ctransmittance curvexe2x80x9d) of this dielectric multilayer filter has a sharp peak; as a result, the filter substantially provides the maximum transmission peak. Consequently, it is possible to improve the transmittance of the optical output signal.
Also, for realizing the variable optical wavelength filter according to the first or second aspect of the present invention, the optical wavelength filtering portion may preferably comprise an actuator for converting the control signal to power and having a shaft for moving this optical wavelength filter based on this power. For example, this actuator may cause the optical wavelength filter to rotate and vary the specific wavelength discussed above; the axis of rotation is the center line of the optical wavelength filter, orthogonal to the crossover line of the incident plane at which the optical input signal is incident to the optical wavelength filter and the filter surface of the optical wavelength filter. Also, for example, this actuator may move this optical wavelength filter within a plane perpendicular to the direction at which the optical input signal is incident to the optical wavelength filter and change the abovementioned specific wavelength.